Mitochondria are centers of metabolism and signaling whose function is essential to all but a few eukaryotic cell types. General dysfunction of these organelles is implicated in a wide range of inborn errors of metabolism, and in an increasing number of common human diseases. Elucidation of the biochemical functions of these disease-related proteins has become a bottleneck in understanding mitochondrial pathophysiology. This proposal initially focuses on one such protein, ADCK3, a predicted kinase with a poorly understood connection to the biosynthesis of coenzyme Q, an integral part of the mitochondrial electron transport chain. Mutations in this protein can cause a form of cerebellar ataxia, one of numerous mitochondrial respiratory chain disorders. More broadly, ADCK3 is part of a large and ancient family of predicted atypical kinases-the UbiB family-that has no structural or functional characterization reported to date. This proposal will establish the fundamental enzymatic properties of this archetypal member of the UbiB family, identify its direct endogenous substrate(s), and test the hypothesis that it functions to remodel the protein infrastructure of th inner mitochondrial membrane through alterations in the local phospholipid environment. Leveraging the depth and breadth of expertise available through the investigators of this program project, the proposed work incorporates a diverse range of tools to address these goals, including: a robust, high mass-accuracy lipidomics platform and yeast, mammalian cell culture and mouse knockout models of ADCK3 deficiency. As the characterization of ADCK3 progresses, the approaches and techniques established here will subsequently be applied to the other ADCK kinases that are generally conserved from yeast through metazoans. These kinases include four in human mitochondria (ADCK1, 2, 4 and 5), which have now been associated both with lipid metabolism and with specific cancers, including glioblastoma multiforme (GBM) and estrogen receptor-positive (ER+) breast tumors. Thus, in addition to the direct insights that will be generate regarding the specific function of ADCK3, this work sets the stage for an extensive analysis of a unique mitochondrial kinase family of mounting biomedical importance.